[PULL,01/12] target/i386: kvm: Demand nested migration kernel capabilities only when vCPU may have enabled VMX

diff --git a/target/i386/cpu.h b/target/i386/cpu.h
index 05393cf..8b3dc55 100644
--- a/target/i386/cpu.h
+++ b/target/i386/cpu.h
@@ -1877,6 +1877,28 @@  static inline bool cpu_has_vmx(CPUX86State *env)
     return env->features[FEAT_1_ECX] & CPUID_EXT_VMX;
 }
 
+/*
+ * In order for a vCPU to enter VMX operation it must have CR4.VMXE set.
+ * Since it was set, CR4.VMXE must remain set as long as vCPU is in
+ * VMX operation. This is because CR4.VMXE is one of the bits set
+ * in MSR_IA32_VMX_CR4_FIXED1.
+ *
+ * There is one exception to above statement when vCPU enters SMM mode.
+ * When a vCPU enters SMM mode, it temporarily exit VMX operation and
+ * may also reset CR4.VMXE during execution in SMM mode.
+ * When vCPU exits SMM mode, vCPU state is restored to be in VMX operation
+ * and CR4.VMXE is restored to it's original value of being set.
+ *
+ * Therefore, when vCPU is not in SMM mode, we can infer whether
+ * VMX is being used by examining CR4.VMXE. Otherwise, we cannot
+ * know for certain.
+ */
+static inline bool cpu_vmx_maybe_enabled(CPUX86State *env)
+{
+    return cpu_has_vmx(env) &&
+           ((env->cr[4] & CR4_VMXE_MASK) || (env->hflags & HF_SMM_MASK));
+}
+
 /* fpu_helper.c */
 void update_fp_status(CPUX86State *env);
 void update_mxcsr_status(CPUX86State *env);
diff --git a/target/i386/kvm.c b/target/i386/kvm.c
index ec7870c..4542f0f 100644
--- a/target/i386/kvm.c
+++ b/target/i386/kvm.c
@@ -128,6 +128,11 @@  bool kvm_has_adjust_clock_stable(void)
     return (ret == KVM_CLOCK_TSC_STABLE);
 }
 
+bool kvm_has_exception_payload(void)
+{
+    return has_exception_payload;
+}
+
 bool kvm_allows_irq0_override(void)
 {
     return !kvm_irqchip_in_kernel() || kvm_has_gsi_routing();
@@ -1342,7 +1347,6 @@  static int hyperv_init_vcpu(X86CPU *cpu)
 }
 
 static Error *invtsc_mig_blocker;
-static Error *nested_virt_mig_blocker;
 
 #define KVM_MAX_CPUID_ENTRIES  100
 
@@ -1653,22 +1657,6 @@  int kvm_arch_init_vcpu(CPUState *cs)
                                   !!(c->ecx & CPUID_EXT_SMX);
     }
 
-    if (cpu_has_vmx(env) && !nested_virt_mig_blocker &&
-        ((kvm_max_nested_state_length() <= 0) || !has_exception_payload)) {
-        error_setg(&nested_virt_mig_blocker,
-                   "Kernel do not provide required capabilities for "
-                   "nested virtualization migration. "
-                   "(CAP_NESTED_STATE=%d, CAP_EXCEPTION_PAYLOAD=%d)",
-                   kvm_max_nested_state_length() > 0,
-                   has_exception_payload);
-        r = migrate_add_blocker(nested_virt_mig_blocker, &local_err);
-        if (local_err) {
-            error_report_err(local_err);
-            error_free(nested_virt_mig_blocker);
-            return r;
-        }
-    }
-
     if (env->mcg_cap & MCG_LMCE_P) {
         has_msr_mcg_ext_ctl = has_msr_feature_control = true;
     }
@@ -1683,7 +1671,7 @@  int kvm_arch_init_vcpu(CPUState *cs)
             if (local_err) {
                 error_report_err(local_err);
                 error_free(invtsc_mig_blocker);
-                goto fail2;
+                return r;
             }
         }
     }
@@ -1752,8 +1740,6 @@  int kvm_arch_init_vcpu(CPUState *cs)
 
  fail:
     migrate_del_blocker(invtsc_mig_blocker);
- fail2:
-    migrate_del_blocker(nested_virt_mig_blocker);
 
     return r;
 }
diff --git a/target/i386/kvm_i386.h b/target/i386/kvm_i386.h
index 3057ba4..06fe06b 100644
--- a/target/i386/kvm_i386.h
+++ b/target/i386/kvm_i386.h
@@ -35,6 +35,7 @@ 
 bool kvm_allows_irq0_override(void);
 bool kvm_has_smm(void);
 bool kvm_has_adjust_clock_stable(void);
+bool kvm_has_exception_payload(void);
 void kvm_synchronize_all_tsc(void);
 void kvm_arch_reset_vcpu(X86CPU *cs);
 void kvm_arch_do_init_vcpu(X86CPU *cs);
diff --git a/target/i386/machine.c b/target/i386/machine.c
index 704ba6d..ac2d1d1 100644
--- a/target/i386/machine.c
+++ b/target/i386/machine.c
@@ -7,6 +7,7 @@ 
 #include "hw/isa/isa.h"
 #include "migration/cpu.h"
 #include "hyperv.h"
+#include "kvm_i386.h"
 
 #include "sysemu/kvm.h"
 #include "sysemu/tcg.h"
@@ -232,10 +233,25 @@  static int cpu_pre_save(void *opaque)
     }
 
 #ifdef CONFIG_KVM
-    /* Verify we have nested virtualization state from kernel if required */
-    if (kvm_enabled() && cpu_has_vmx(env) && !env->nested_state) {
-        error_report("Guest enabled nested virtualization but kernel "
-                "does not support saving of nested state");
+    /*
+     * In case vCPU may have enabled VMX, we need to make sure kernel have
+     * required capabilities in order to perform migration correctly:
+     *
+     * 1) We must be able to extract vCPU nested-state from KVM.
+     *
+     * 2) In case vCPU is running in guest-mode and it has a pending exception,
+     * we must be able to determine if it's in a pending or injected state.
+     * Note that in case KVM don't have required capability to do so,
+     * a pending/injected exception will always appear as an
+     * injected exception.
+     */
+    if (kvm_enabled() && cpu_vmx_maybe_enabled(env) &&
+        (!env->nested_state ||
+         (!kvm_has_exception_payload() && (env->hflags & HF_GUEST_MASK) &&
+          env->exception_injected))) {
+        error_report("Guest maybe enabled nested virtualization but kernel "
+                "does not support required capabilities to save vCPU "
+                "nested state");
         return -EINVAL;
     }
 #endif